Commercial olefin hydration processes react water with an olefin over an acid catalyst to produce alcohols. These processes also produce a co-product ether. These ethers are dialkyl ethers of the olefin being fed. By way of example, when propylene is the feed, the ether is di-isopropyl ether (IPE), and for butenes it is di-secondary butyl ether (SBE). The yield of ether produced is determined by the type of hydration process used and the process conditions. For processes that produce isopropyl alcohol (IPA) and secondary butyl alcohol (SBA), ether co-production typically ranges from about 1% to about 15% by weight. Indirect hydration processes, such as those using sulfuric acid, typically produce from about 5% to about 15% by weight ether as a co-product. Direct hydration process which use high purity olefin feeds over solid acid catalyst, such as acid resins, produce less ether but still in the 1 to 7% range. The production of ether reduces the yield of the alcohol; therefore, when alcohol is the desired product it would be advantageous to minimize the amount of ether co-product.
The reaction to produce ether is an equilibrium reaction, e.g., for C3's (Rxn I) propylene reacts with isopropyl alcohol, over an acidic catalyst, in a reversible reaction to form IPE.

However, in indirect hydrolysis systems sulfuric acid is used to provide the acidity and forms intermediate esters with the olefin and alcohols. It is also a reversible equilibrium process. Rxn 2-4 below show the reactions of C3H6 and IPA that form IPE. In order to react recycled IPE there has to be enough sulfuric acid available to reform the alkyl sulfate, the reverse of Rxn 4.

Several methods have been utilized to minimize ether co-product yield. It is possible to minimize ether by reducing the severity of the reaction or to run the reaction in a dilute environment. Another way is to utilize a production method that minimizes ether yield such as in direct hydration. Ether can also be thermally decomposed back to the olefin, it can be decomposed back to the alcohol and an olefin, and it can be recycled back to the hydration reaction step to minimize further production of ether.
The present inventors have discovered methods to maximize ether reversion when it is returned or recycled to the hydration reaction section of the process. These improvements are particularly important for hydration processes that produce >7% by weight ether co-product and/or where the economic value of the ether is lower than the olefin feed stock.
It has been observed that the amount of ether that can effectively be recycled is limited by typical olefin hydration process conditions. In indirect hydration processes conditions are usually mild, i.e., moderate temperatures and pressures, to minimize side reactions. At those conditions the rate of reversion of ether is slow and there is a limited recycling capacity that is available. In fact, only on the order of 20 to 40% can be recycled under those conditions. In a gas absorption process, when ether is fed in excess of this rate it will build up as a liquid phase in the process. This results in unsteady unit operation with ether phase accumulation and results in hydration process unit upsets. Improvements to the recycling process are therefore needed. We have found that by adopting the improvements described here it is possible to greatly increase capability for recycle of ether and hence the yield of the desired alcohol.
Recycle per se has been described in the literature for hydration processes. U.S. Pat. No. 4,579,984 describes a recycle process for a direct hydration process using acid catalyst where the dialkyl ether is introduced into the reaction mixture at a set distance from the outlet of the reactor.
U.S. Pat. Nos. 4,405,822 and 5,569,789 deal with recycling ether to an ether hydrolysis reactor to produce IPA at elevated temperatures and pressures. These are also direct hydration processes and the conditions are not similar to milder indirect hydration conditions.
Recycling ether back to a sulfuric acid system is described in U.S. Pat. No. 2,533,808 and more recently is mentioned in U.S. Pat. No. 7,399,891. However, a recycle according to the present invention is not suggested in the prior art, as far as is known by the present inventors.
The ability to recycle ether into indirect hydration processes has been, heretofore, limited by the mild process conditions that are utilized, by the hydrophobic nature of ether, and by relatively slow absorption rates. These limitations limit the amount of ether that can be normally recycled to only 20-40% of the ether that is produced. This results in limited flexibility to control the ether yield. By careful study of the ether absorption process we have discovered methods to increase absorption of ether up to, in embodiments, 100% or near 100% of that produced. Implementation of these process configurations and conditions will improve the operational stability and yield flexibility of indirect hydration processes.
The present inventors have surprisingly discovered that providing an ether recycle that is substantially vaporized into the reaction vessel(s) substantially overcomes one or more of the aforementioned problem and improved alcohol yield.